Cylinder to cylinder variations in combustion associated with air/fuel ratio imbalances may occur in engines due to various factors. For example, cylinder-to-cylinder air/fuel ratio imbalances may occur due to cylinder-to-cylinder variation in intake valve depositions, plugged EGR orifices, electrical faults, air leaks, and/or shifted fuel injectors, etc.
In some approaches, air/fuel imbalances may be monitored using a proportional oxygen sensor (e.g., UEGO) to calculate an amount of imbalance by determining a high frequency component of the sensor signal related to cylinder-to-cylinder deviation. In such an approach, cylinder air/fuel imbalance may be estimated by accumulating the amount of cylinder-to-cylinder deviations per calibratable RPM window, for example.
However, the inventors herein have recognized that such approaches may not provide individual cylinder capability and may not differentiate air mass delivery route faults from fuel pass delivery faults thereby leading to errors in individual cylinder imbalance detection. Further still, in such approaches, sensor contamination and aging may diminish monitor capability over time by lessening sensor frequency response.
In other approaches, cylinder air/fuel imbalances may be monitored using crankshaft acceleration signal processing to detect cylinder-to-cylinder variations in combustion associated with air/fuel imbalances. However, such approaches may erroneously identify air/fuel imbalances since fluctuations in crankshaft acceleration may be due to so many other factors, e.g., spark plug fouling, ignition coil malfunction, etc.
In one example approach, in order to at least partially address these issues, a method for monitoring cylinder air/fuel imbalances is provided. The method comprises, identifying a cylinder with a potential air/fuel imbalance based on crankshaft accelerations generated by a applying a predetermined pattern of rich, lean, and stoichiometric conditions in the cylinder while keeping an overall exhaust mixture at stoichiometry. For example, the predetermined pattern of rich, lean, and stoichiometric conditions in the cylinder may be selected so that lean combustions are compensated by rich combustions in other cylinders leading to a common exhaust passage.
In this way, emission and driveability impact together with noticeable RPM disturbances during imbalance monitoring may be reduced since the exhaust is kept at stoichiometry during the monitoring. Further, air/fuel causality of maldistribution may be identified on a cylinder to cylinder basis to identify the sign or direction (e.g., rich/lean) of the imbalance along with an amount of correction to mitigate the emissions effects of an individual cylinder imbalance. Such an approach is made possible at least in part due to the pre-selection of the applied pattern. The a priori knowledge thus enables the system to compensate for air and/or fuel path errors.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.